In the field of industrial lighting, the aim of the lighting designer is to place as much light output from the lighting fixture in as precise a controlled area, or workplane, as possible. In performing this lighting design function, this operation of customizing the lighting direction includes optimizing the coefficient of utilization; that is, given that the coefficient of utilization is a ratio value indicative of the amount of light that can be directed to the workplane intended, it is most desirable to achieve the highest value coefficient of utilization. In attempting to achieve this optimum utilization of light from the lighting fixture, it is known to use a variety of light controlling lens members which can exhibit either reflective, refractive or a combination of refractive and reflective properties. An example of one reflector for use with a luminaire can be found in U.S. Pat. No. 3,457,399 issued to Milroy on Jul. 22, 1969. FIG. 7 of this reference illustrates an aluminum reflector for which it is known that approximately 20% of the lamp lumens is attenuated by the reflector.
An example of a specific lighting fixture designed to achieve a precisely controlled distribution of light to a particular work area can be found in U.S. Pat. No. 3,705,303 issued to Willis Jr. et al on Dec. 5, 1972 and assigned to the same assignee as the present invention. In this patent, the lighting fixture includes a conventional light source and ballast arrangement in combination with a cover member having a fluted upper reflector portion and an outwardly flared lower refractor portion. The luminaire in this patent, though effective for its intended purpose of controlling light output to a specific, low mounting application, is limited in that an aluminum reflector is used to reflect about 60% of the light output which is then directed through a lens member at the bottom end. Additionally, an aluminum reflector as discussed in this patent further suffers from the attenuation characteristics previously discussed.
Another example of a luminaire having specific light directing capabilities can be found in U.S. Pat. No. 4,118,763 issued to Osteen on Oct. 3, 1978 and assigned to the same assignee as the present invention. In this patent, the luminaire includes a globe member disposed about the light source and which is constructed of a light transmissive material having a number of different prismatic surfaces which are effective in combination with the shape and mounting of the cover, such that the light is output in various directions including the upward direction. The application of the embodiments of this patent is one in which it is desired to provide a significant amount of uplighting and accordingly, this patent relies primarily on the reflective properties of the prismatic surfaces formed on the cover member.
In an industrial application such as the lighting of a warehouse or manufacturing floor having equipment which extends upward a significant distance from the actual floor or equipment height level, it is desirable to control the light output such that a downward directed light output is achieved using a predominantly refractive light transmissive cover which allows light output to be directed in a substantially sideways direction in addition to the downward direction. Another example of a luminaire having specific light directing capabilities is a product marketed as a Vertiflex Industrial luminaire provided by the Crouse-Hinds Division of Cooper Lighting which has a place of business in Vicksburg, Miss. This luminaire utilizes a predominantly refractive fixture which results in a very wide beam lighting distribution having significant uplighting characteristics. Though exhibiting a high efficiency, the Super Watt luminaire exhibits poor lumen utilization since most of the lamp lumens are directed at high angles.
Accordingly, it would be desirable to provide a lighting fixture which achieved sufficient illumination at heights typically found in an industrial or warehouse environment and whereby such illumination was achieved using refractive elements that could direct a substantial portion of the light output in a downward direction having an angle of dispersion less than 60 degrees relative to the nadir when measured near the luminaire. It would be further advantageous if the light source would be disposed relative to the lens member in a manner to maximize the amount of available lamp lumens for the necessary controlled illumination. It would also be advantageous if, in addition to the applicability to a high bay configuration, this luminaire could be utilized in a low bay type of environment without modification to the luminaire.